1. Field of the Invention
The present invention relates generally to a hybrid automatic repeat request (ARQ) system, and more particularly, to an adaptive hybrid ARQ method and apparatus that minimizes transmission delay by measuring an error degree of a frame in which error correction fails at a receiving terminal and retransmitting a parity bit in accordance with the error degree at a transmitting terminal, and a method of transmitting and receiving data in an adaptive hybrid ARQ system.
2. Description of the Related Art
A hybrid ARQ system combines a basic ARQ technique, which requests retransmission from a transmitting terminal by detecting an error from a received signal, with a forward error correction (FEC) channel coding technique, which overcomes deterioration of the channel. The hybrid ARQ system is designed to increase the overall throughput of a wireless communications system in time-varying channels. The hybrid ARQ system uses three hybrid ARQ methods: type I, type II, and type-III. The hybrid type II ARQ method has been proposed to solve problems of the hybrid type I ARQ method. In this method, ARQ is adaptively used for a channel having a dynamic bit-error-rate depending on the state of the channel. Two codes are used in the hybrid type-II ARQ method. One code is used to generate an n-bit code word D by using an error detection code C0 of (n, k) in a message bit k. The other code is used to generate a 2 k-bit parity block P(D) by using an error detection and correction code C1 of (2 k, k) in a parity bit k. A new code word F=(D,P(D)) is generated using the code word D and the parity block P(D) that are respectively generated using the two codes C0 and C1. When a block D of a new code word F is transmitted and an error is detected at a receiver side using the error detection code C0, the received block D is stored in a buffer, and retransmission is requested from a transmitter side. Upon this request, if the transmitter side retransmits only the parity block P(D), when a syndrome is calculated and an error is not detected at the receiver side, the code word D can be recovered using the parity block P(D). However, if the error is detected at the receiver side, the error is corrected by means of the error detection and correction code C1 using the block D stored in the buffer and the parity block P(D). If error correction fails in the two steps, the block D stored in the buffer is discarded and the parity block P(D) instead is stored in the buffer, and then, retransmission is requested from the transmitter side. The transmitter side that has been requested of retransmission retransmits the block D, instead of the parity block P(D), so as to repeat the error detection and correction procedure. Even when the block D is received free of errors or an error is detected, the above procedure is repeated until a code word that can correct the error successfully is received.
An example of the hybrid type-II ARQ method is illustrated in FIG. 1. Referring to FIG. 1, a transmitter 110 transmits a data frame comprising a parity bit 121 and a data bit 122 that are generated using a code having a high coding rate during initial transmission. A receiver 130 receives the data frame and performs channel decoding of the received data frame. If channel decoding fails, the receiver 130 transmits a negative acknowledgement (NACK) message 123 to the transmitter 110. When receiving the NACK message 123, the transmitter 110 transmits a parity frame comprising a parity bit 124 that is generated using a code having a lower coding rate than an initial coding rate.
Next, the receiver 130 corrects an error that occurs in a previous message using the transmitted parity bit 124. If this error correction is successful, the receiver 130 transmits an ACK message 127 to the transmitter 110. If the error correction is not successful, the receiver 130 transmits a NACK message 125 to the transmitter 110. When receiving the NACK message 125, the transmitter 110 transmits a parity frame comprised of a parity bit 126 that is generated using a code having a lower coding rate than a previous coding rate. The receiver 130 corrects an error that occurs in a previous message using the transmitted parity bit 126. If this error correction is successful, the receiver 130 transmits the ACK message 127 to the transmitter 110.
In the above-described method, a message bit of a previously transmitted frame is reused so that improvement of the overall throughput is achieved. However, if frame transmission fails, a parity bit is transmitted by reducing a coding rate stepwise. As such, transmission delay TD1 becomes longer. Ultimately, due to the longer transmission delay, limitation of a transmission time on real-time data, such as voice, cannot be satisfied. In addition, data must be stored in a transmission buffer and a receiving buffer during several retransmissions, resulting in buffer overflow.
Basic technologies for hybrid ARQ are described in the articles “A Hybrid ARQ Scheme With Parity Retransmission for Error Control of Satellite Channels,” by S. Lin and P. S. Yu, IEEE Transaction on Communications, Vol. 30, pp. 1701–1719, July 1982, “Hybrid ARQ Protocol for Real-time ATM Services in Broadband Radio Access Networks,” by C. W. Ahn, W. S. Kang, CH. Kang and C. G. Kang, IEEE TENCON 99, Vol. 2, pp. 1379–1382, 1999 and “Simple Hybrid Type II ARQ Technique Using Soft Output Information,” by P. Coulton, C. Tanriover, B. Wright and B. Honary IEE Electronic letters, Vol. 36, No.20, pp. 1716–1717, September 2000.